Electrolytic capacitors are normally formed with a conductive anode sheet and a conductive cathode sheet with paper separator sheet between to form a thin sandwich. An electrolyte impregnated in the separator sheet functions as the cathode in continuity with the cathode sheet, with an oxide layer on the anode functioning as a dielectric.
In cylindrical capacitors, the sandwich is tightly rolled along with another separator sheet; in flat capacitors, multiple sandwiches are stacked with insulating sheets between. Where a high capacitance per unit volume is needed, highly etched anode foil has been employed. Highly etched foil has a deeply microscopically contoured surface having a total surface area much greater than the flat area of a given sheet. A surface area coefficient, the ratio of the microscopic true surface area to the macroscopic apparent area, may be as high as 100 in current devices, permitting miniaturization of capacitor components.
In manufacturing such miniature capacitors, a dielectric oxide layer is formed on the etched anode sheets to function as a dielectric for the capacitor. However, this embrittles the sheet, leading to several disadvantages. In rolled capacitors, the brittle sheet does not readily roll to a tight radius; too tight of a radius leads to unacceptable cracking and fractures, and too loose of a roll radius leaves a large vacant core in the finished capacitor, increasing component volume. In both rolled and flat capacitors, cutting the desired shapes from the etched and oxidized master anode sheet can lead to fractures and chipping at the edges of the cut sheets. These may contribute to shorts, or require additional tolerances to be built into the component design, increasing volume. In some cases, sharp edge fragments may penetrate the separator material and cause a shorting component failure.
It is also difficult to connect the anode sheet to a component lead, as conventional welding and compressive techniques may damage the fragile material. Flat capacitors in particular have extending tabs that must flex in order to be brought together for welding; brittle material is poorly suited for this. Even in routing handling during manufacturing, oxidized anode layer sheets are vulnerable to damage that reduces manufacturing yields.
Past efforts to reduce these problems have employed selectively etched master sheets, with critical regions masked to prevent etching, and thereby limiting brittleness after oxide formation. However, such efforts increase the cost of procuring anode sheet material, impede design changes, and complicate inventory management.
The present invention overcomes the limitations of the prior art by providing a method of manufacturing a capacitor by highly etching a foil sheet, and at selected relief portions of the sheet, reducing the surface area coefficient. An anode sheet is cut from the foil sheet and assembled with other capacitor components. The surface area coefficient may be reduced by coining or mechanically compressing at the selected relief portions, and an oxide layer may be formed on the sheet after coining.